1. Field of the Invention
The present invention relates to carbon dioxide gas generating compositions which produce carbon dioxide gas with good stability in gas phases of widely varying relative humidities and which are suitable for use as agents for preserving the freshness of vegetables, fruits, etc.
2. Description of the Prior Art
Existing carbon dioxide gas generating compositions are used in baking powders, powders for use of refreshing beverage, artificial tooth cleaning agents, bathing agents, fire extinguishing agents or the like. These generating compositions are adapted to undergo a reaction within a short period of time, e.g., within several minutes, when heated or placed into water.
Especially for use in bathing agents which are used as admixed with a large quantity of water, the composition which is in the form of a mixture of acid and carbonate is prepared as granules or tablets so as to ensure effective contact between the acid and the carbonate. For this reason, the composition is prone to reaction during preservation, so that several proposals have been made to render such compositions preservable for a prolonged period of time.
These proposals include contact of the composition as shaped with carbon dioxide gas immediately before packaging to partly convert the carbonate into a hydrogencarbonate for the removal of free water (JP-A-141609/1986), addition of synthetic zeolite which has high ability to remove moisture (JP-B-9241/1989), use of tartaric acid only as the acid to be used and use of anhydrous sodiumcarbonate for taking up water as crystal water (JP-A-7246/1981), and use of anhydrous potassium carbonate in combination with the composition to afford improved preservability (JP-A-172810/1990).
However, the generating compositions of these proposals are not intended to produce carbon dioxide in a gas phase and fall to meet the conditions under which such compositions are to be used in a gas phase.
On the other hand, compositions ale proposed which are adapted to produce carbon dioxide in a gas phase by using an acid and further a hygroscopic substance in combination with a carbonate (JP-A-20270/1990). However, these proposed compositions start a reaction when the components are merely mixed together, therefore can not be stored as generating compositions and are inconvenient to handle because the components must be mixed together at the site of use.
Some compositions have also been proposed which are intended to produce carbon dioxide gas for a long period of time. These include one to be buried in soil for use as a fertilizer (JP-B-38524/1981), and a composition as absorbed by kieselguhr, which is further coated with gelatin to retard the reaction (JP-B-13367/1969).
The former evolves ammonia, is therefore difficult to use for foods, and is used under different conditions since it is used as a fertilizer as buried in earth. The latter generates little or no gas at a relative humidity of up to 80%, requires a cumbersome treatment and is not effectively usable.
It has been found that carbon dioxide of high concentration is extremely effective for preserving the freshness of vegetables, fruits and perishable foods, especially vegetables and fruits such as broccoli, ginggengcai and welsh onion. To provide an effective environment for preserving freshness of vegetables and fruits, it is desired to develop carbon dioxide generating compositions. In this case, compositions are desirable which generate carbon dioxide gas continuously over a distribution period or storage period in accordance with widely varying humidity conditions as involved in a gas phase of high humidity in the interior of a vegetable- or fruit-containing container wherein water vapor condensate is formed on the inside panel and a gas phase of low humidity wherein vegetables or fruits which awe less likely to release water vapor are present. Nevertheless, none of existing carbon dioxide gas generating compositions conform to these conditions and ensure high safety.